Polyolefins are typically produced as a copolymer of an olefin monomer and an olefin comonomer. For example, linear low density polyethylenes (hereinafter, referred to as “LLDPE” in some cases) are usually produced by copolymerizing ethylene (the olefin monomer) with an alpha-olefin such as 1-hexene (the olefin comonomer). Typically, polymerization typically occurs in the presence of a transition metal compound catalyst such as a Ziegler-Natta catalyst, chromium based catalyst, and/or a metallocene catalyst. Polymerization processes generally include solution polymerization, slurry polymerization and gas phase polymerization. The polymers, and in particular, polyethylene copolymers, produced by any of the foregoing polymerization processes possess a composition distribution that is largely dependent upon the type of catalyst used. A “broad” composition distribution means that of the polymer chains produced, the amount of comonomer incorporated into each polymer chain varies within a broad range, whereas a “narrow” composition distribution is one where the comonomer is incorporated evenly among the polymer chains. This characteristic is often referred to as CDBI (Composition Distribution Breadth Index) known to those skilled in the art.
It is desirable in any commercial polymerization process to have the option of tailoring the process such that the resultant polymer has the desired characteristics. One such tailorable characteristic of a polymer is its CDBI value. Many catalyst systems comprising a metallocene are typically known to produce narrow CDBI polymers such as polyethylene, or stated another way, polymers having a CDBI of greater than 50 or 60%, the percentage referring to the weight percent of the polymer molecules having a comonomer content within 50% of the median total molar comonomer content, such as described in WO 93/03093. On the other hand, certain metallocenes, such as those hafnocenes disclosed in U.S. Pat. No. 6,242,545 and WO 2004/000919 are known to produce polyethylenes having a broad CDBI, or less than 50 or 40%. The CDBI of a polymer, among other parameters, are known to influence the properties and thus usefulness of the polymer for certain applications. What would be desirable, however, is a method of controlling the CDBI of a polymer, in particular, polyethylene, without having to change the type of catalyst being used.